Add module "tuple" to libstd

[kud1ing]

• Extract fst() and snd() from "rustdoc/util.rs" and put it in a "tuple.rs" in libstd.
  • Maybe find better names like "first" and "second".
• Maybe also add a swap(), see http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Tuple.html

See ab86642#comments

[kud1ing]

Done